It is known that upon color development of a silver halide color photographic material, coupler reacts with an oxidation product of an aromatic primary amine color developing agent to form a dye such as indophenol, indoaniline, indamine, azomethine, phenoxazine, phenazine, or the like, thereby a color image is formed.
In this system, color reproduction is usually accomplished with the subtractive color process and silver halide emulsions selectively sensitive to blue, green and red, and yellow, magenta and cyan color image forming agents (couplers) in a complementary relationship therewith are employed.
The properties of color photographic light-sensitive materials have recently improved. For example, improvement in image qualities such as color reproducibility, sharpness or graininess, etc., improvement in color image preservability to heat, light or humidity, and increase in sensitivity, etc. In the process of the improvement in these photographic properties, progress in the molecular design of the color image forming couplers has carried out a great role because there is a relationship between the properties of couplers and the properties of photographic light-sensitive materials. More specifically, when an absorption spectrum of a dye formed upon the coupling reaction of coupler with the oxidation product of a developing agent is broad or has a subsidiary absorption, color turbidity occrus which results in damage of the color reproducibility. Further, since the amount of couplers incorporated into the emulsion layer can be reduced with an increase in the amount of dyes which are formed per an unit weight of the couplers, the thickness of the layer can be reduced which results in improvement in sharpness. Moreover, the image preservability is improved as the stability of the dyes formed from couplers to heat, light or humidity is increased. Furthermore, as the rate of the coupling reaction of the coupler with the oxidation product of the developing agent is larger, the development of silver occurrs more efficiently, which results in an increase in the sensitivity of the photographic light-sensitive material.
Various kinds of couplers have been proposed for the purpose of improvement of the photographic properties described above. For example, 5-pyrazolone type couplers which form axomethine dyes having a little subsidiary absorption on the short wavelength side are disclosed in Journal of Chemical Society, Perkin I, page 2047 (1977). In U.S. Pat. No. 4,248,961, bis-type yellow couplers are disclosed in order to decrease the molecular weight of the couplers necessary to form a certain density of dyes. Further, couplers which provide color images of improved fastness are disclosed in U.S. Pat. Nos. 3,311,476 and 4,124,396. Moreover, couplers having a high color forming property which are advantageous for the purpose of achieving high sensitivity are disclosed in U.S. Pat. Nos. 4,296,199 and 4,301,235.
As described above, it is recognized that the properties of recently provided couplers are improved to some extent. However, further improvement in their properties is difficult due to the restriction on the molecular design of couplers. This is because the properties of the dyes formed from couplers are determined by the molecular structure of the couplers, which is a feature that the above described couplers known heretofore commonly possess. It is natural when considering that the azomethine dyes are formed upon the coupling reaction of the couplers with the oxidation products of developing agents.
On the other hand, this makes an unavoidable limitation in the molecular design of the couplers and causes several contradictions. For example, when the molecular structure of the coupler is designed only taking into consideration increase in the coupling speed, sometimes the absorption spectrum of the dye formed from the coupler thus-obtained is different from the desired color or a subsidiary absorption of the dye is large. As a result, the coupler can not be practically used. Further, a coupler having a low speed of coupling with the oxidation product of a developing agent can not be employed even if it is excellent in chemical stability and can form a dye of good fastness. On the contrary, a coupler which forms a dye having a sharp spectral absorption curve whithout subsidiary absorption can not be utilized in photographic light-sensitive materials when the coupler pe se is unstable or the dye formed therefrom is not stable. Moreover, when a dye having a large molecular extinction coefficient (.epsilon.) which is advantageous for reducing the thickness of the emulsion layer is desired, it is impossible to utilize the dye if it can not be formed upon the coupling reaction thereof with the oxidation product of a developing agent.
These disadvantages are based on the utilization of dyes formed upon the coupling reaction of three kinds of couplers, i.e., yellow, magenta and cyan couplers with the oxidation products derived from the kind of developing agent. That is, the degree of freedom on the selection of the structure of dye is small, and thus the degree of freedom on the selection of the coupler is small. It is impossible, therefore, that each of the various properties required for the couplers is independently controlled so as to obtain the optimum conditions.
In other words, in the case of color image forming couplers heretofore known, it is obliged to compromise various photographic properties due to the several restrictions described above when a coupler having the desired properties is selected.